To arrive at a final top-\(k\)  ranking, the original DPR paper generated top \(k^{\prime }\)  results from DPR and top \(k^{\prime }\)  results from BM25 (where \(k^{\prime } > k\) ), before considering the union of the two result sets and combining the scores to arrive at the final top \(k\) .
[1]} set \(k^{\prime }=2000\) , but after some preliminary experimentation, we decided to fix \(k^{\prime }=1000\)  in our experiments since it is a more common setting in information retrieval experiments (for example, \(k=1000\)  is the default in most TREC evaluations).
